There are many circumstances where it is necessary or desirable to determine the geo-location of an RF emitter. Applications include search and rescue (SAR) operations that locate an emergency signal and military signal intelligence operations for locating communications facilities.
Various single platform systems have been developed for determining the geo-location of an RF emitter. While single platform systems are inexpensive when compared to multiple platform systems, single platform systems are incapable of executing a simultaneous, coherent collection of an RF emitter signal. FIG. 1A shows a typical single platform system which triangulates the position of an RF emitter by comparing data points taken at different times (t1, t2) from the same platform P1. A larger baseline, or the distance between the points where the signals are collected, is desirable to increase the accuracy of the system. However, the single platform must travel the distance of the baseline, consuming time to determine the precise geo-location of the target emitter. In instances where the target emitter is mobile, this limitation is critical.
Using a Search And Rescue (SAR) mission as an example, precision single platform geo-location is accomplished by flying from one position to another to triangulate on the RF emitter signal. The time consumed during this process can be fatal when rapid response is required for a life-threatening situation.
Cooperative geo-location systems can quickly triangulate on an RF emitter by comparing simultaneous signals collected from multiple platforms (P1, P2). See FIG. 1B. While these designs are able to minimize the duration of time required for triangulation, multiple platform designs require an increased number of assets and generally require additional coordination efforts, making their implementation cost-prohibitive.
Alternative single platform designs deploy tethered or towed platforms for receiving simultaneous signals. Another solution is to install RTDs at different points on the airplane. These prior art designs operate under a similar principle as other multiple platform designs, collecting simultaneous signals at different angles. Such designs have limited success in determining an azimuth angle, constrained by the length of the cable or of the airplane which limits the baseline. These systems are even less useful for determining an elevation angle because the baseline with respect to elevation is generally limited to the vertical height of a plane. Further, a tethered and towed device will travel at the same velocity as the primary platform and thus is a very poor design for frequency difference of arrival (FDOA) measurements.
Accordingly, there is a need for an improved system for determining the geo-location of an RF emitter with optimal time-response and minimal cost and coordination required.